The "dry mix" packerhead method of concrete pipe manufacture has gained wide acceptance in recent years. The predecessor "wet mix" method required a relatively long term curing period with forms in place and was not conductive to high rates of production. With the "dry mix" packerhead method, a separable external form or jacket is employed and may be removed almost immediately on formation of a pipe therewithin. The "green" formed pipe has sufficient structural integrity to stand upright on its own and may be transported to a curing oven for a substantially shorter curing operation, the jacket being returned to the pipe making machine for the formation of a succeeding pipe.
Generally, the dry mix packerhead method comprises the steps of positioning a separable tubular jacket beneath a rotatable and axially movable packerhead having an associated dry mix delivery means. The packerhead is lowered within the jacket, having an external diameter somewhat smaller than the internal diameter of the jacket, and is drawn slowly upwardly through the jacket. The packerhead is rotated during its upward movement and mix is delivered to its upper surface and discharged radially outwardly to form the pipe in the annular space between the periphery of the packerhead and the jacket. The peripheral surface of the packerhead trowels the internal surface of the pipe and compacts the pipe during its upward rotary movement.
While the packerhead method of pipe manufacture has proven generally satisfactory, it has been found that toroidal and other residual stresses occur in the pipe and this may be particularly troublesome when steel reinforcement members are employed. Such members may also be stressed during pipe formation and significant circumferential displacement of axially extending members may occur. Residual stresses in the wall of the pipe and in the steel reinforcement therein may result in defective and even in a worthless end product.
In overcoming the stress problems encountered, a step known as "redensification" has been employed in the more recent manufacture of concrete pipe. Redensification involves the application of vibratory motion to the pipe for further compaction, elimination of voids, and especially for the relief of residual stresses in the pipe and/or steel reinforcement therein.
U.S. Patent to Trautner U.S. Pat. No. 3,655,842 entitled METHOD OF VIBRATING CORE IN CONCRETE PIPE MAKING MACHINE, illustrates and describes one method and apparatus for concrete pipe redensification. In this patent, a vibrating "redensifying" core is positioned beneath a forming jacket and an associated packerhead and is drawn upwardly through the pipe during packerhead formation of the pipe. Obviously, radial dimensions of the core are critical. If the core is small enough so as to be drawn upwardly through the pipe without marring the pipe internal surface an inferior redensification operation may result. That is, firm engagement of the vibrating core with the wall of the pipe will not be achieved and pipe compaction and stress relief will not occur uniformly throughout the mass of the pipe and the steel reinforcement. If, on the other hand, a larger core is employed, the internal surface of the pipe may be scarred and severely damaged during core entry and withdrawal. Even with a larger core firm engagement between the core and pipe may not be achieved uniformly throughout the area of the pipe inner surface and inconsistent compaction and stress relief may result.
U.S. Pat. Nos. 4,041,118 and 4,042,315, owned by the Assignee of the present application and respectively entitled METHOD AND APPARATUS FOR MAKING CONCRETE PIPE and APPARATUS FOR MAKING CONCRETE PIPE illustrate and describe an improved method and apparatus for concrete pipe redensification. An expandable and contractable vibrating or redensifying core is employed and problems of core entry and withdrawal are overcome. Further, it has been found that pressure engagement of the expanded core with the internal surface of the pipe during vibration results in a much improved redensification operation. Pipe compaction and stress relief is greatly enhanced and a superior end product results. Problems are encountered, however, with the method and apparatus of these patents. The packerhead formed pipe must be lifted from the pipe making machine in its jacket, transported horizontally and then lowered into position about the core. After redensification, the pipe and jacket must be lifted from the core and returned to the pipe making machine for a subsequent finishing operation by the packerhead, this consisting in a final pass of the packerhead with mix added to trowel and finish the pipe interior surface and to fill any void that may occur at the top of the pipe due to slumping during redensification. Obviously, the jacket and the green pipe therewithin must be handled with extreme care and are very heavy and unwieldy elements for transport at high rates of movement. The operation thus entails severe inherent impediments to pipe manufacture at high rates of production.
It is the general object of the present invention to provide a high production concrete pipe making machine of the dry mix packerhead type and which includes improved redensification apparatus adapted for power driven and automatic operation.
A further object of the invention resides in the provision of a machine of the type mentioned wherein heavy forming jackets and pipes therewithin may be moved rapidly and efficiently from one machine station to another without lifting and transporting or otherwise removing the same from the pipe making machine.
A still further object of the invention resides in the provision of the pipe making machine of the type mentioned wherein a radially expandable and contractable vibrating core is provided and adapted for fast and efficient entry and withdrawal from a packerhead formed pipe.
A still further object of the invention resides in the provision of a pipe making machine of the type mentioned wherein automatically operable guide and support means are provided for the vibrating core to insure precise axial alignment of the core with the pipe during entry and withdrawal.
A still further object of the invention resides in the provision of a machine of the type mentioned wherein an improved true circle core expansion and contraction mechanism is provided.